Follow that Ground Station! And double the data throughput using polarization diversity.

Transcription

1 SSC09-VI-8 Follow that Ground Station! And double the data throughput using polarization diversity. Peter Garner, Nigel Phillips, Andrew Cawthorne, Alex da Silva Curiel, Phil Davies, Lee Boland Surrey Satellite Technology Limited Tycho House, 20 Stephenson Road, Surrey Research Park, Guildford, Surrey GU2 7YE United Kingdom ABSTRACT This paper describes the X-Band Antenna Pointing Mechanism to be used for the first time as part of the Payload Downlink Chain on-board the SSTL-300 platform for the upcoming NigeriaSAT-2 mission, due for launch late in NigeriaSAT-2 is a high performance Earth Observation mission designed for a 7.5 year lifetime to achieve 2.5m imagery in a panchromatic waveband along with 5m and 32m imagery in four mutli-spectral channels. The spacecraft will deliver high data throughput on an agile platform, whilst still maintaining high levels of pointing accuracy during downlink opportunities. All of this will be included in a 300Kg satellite. The innovative Antenna Pointing Mechanism developed by Surrey Satellite Technology Limited incorporates a compact circularly polarized antenna with a narrow 3dB beam-width of 25degrees and bore-sight gain of 15dBiC. It also houses the necessary drive electronics and structural elements to provide a nimble, 2-axis antenna solution. The pointing accuracy of the unit is better than 1 degree and the maximum slew rate is better than 20deg/s, at acceleration rates up to 4deg/s/s. Such low-cost and compact yet agile antenna systems have not been used previously on small satellite missions and as such, the Antenna Pointing Mechanism is an enabling technology in the commercialization of small Earth Observation spacecraft missions. Numerous design trades were made along the development path of the Antenna Pointing Mechanism and these are covered in this paper. Additionally a treatment of the use of polarization diversity on the NigeriaSAT-2 mission as a way of doubling the data throughput capability is reported. INTRODUCTION Surrey Satellite Technology Limited (SSTL) are constantly striving to develop cost effective Earth Observation (EO) imaging platforms that are suitable for carrying out commercial missions at much lower cost than traditional large satellite missions. The SSTL- 300 platform is a 300Kg satellite that is designed to meet demanding high-duty cycle imaging applications inside a compact, yet highly agile platform. It will see first use on the upcoming NigeriaSAT-2 mission for the Nigerian National Space Research and Development Agency (NASRDA), due for launch late in NigeriaSAT-2 is a high performance Earth Observation mission designed for a 7.5 year lifetime to achieve 2.5m imagery in a panchromatic waveband along with 5m and 32m imagery in four mutli-spectral channels. The spacecraft will deliver high data throughput on an agile platform, whilst still maintaining high levels of pointing accuracy during downlink opportunities. This platform can be delivered within timescales of months, inside limited budgets, and could be batch launched in constellations to provide higher temporal coverage. The Payload Downlink Chain (PDC) supports high speed data storage, processing, and data down-linking in X-Band. The chain is scalable to suit customer specific mission requirements. There are three components to the PDC as shown in Figure 1. Figure 1: Payload Downlink Chain Components These are; the High Speed Data Recorder (HSDR) which deals with the data storage and processing aspects of the imaging data from the payload, the X- Band Transmitter (XTX) which modulates the data onto an RF carrier signal and provides coding gain along with RF signal amplification and filtering and finally the Antenna Pointing Mechanism (APM) which focuses the RF energy into a high-gain spot-beam that can be mechanically steered in two axes to track the position of the Ground Station during passes even if the satellite performs high slew-rate manoeuvres during a pass. This paper reports on the development of the APM at SSTL and the use of polarization diversity as a simple lowcost method of doubling the possible data throughput for a typical EO mission in Low Earth Orbit. Garner 1 23 rd Annual AIAA/USU

2 APM DEVELOPMENT A unique aspect of the downlink chain is the implementation of a low cost APM. The APM incorporates a circularly polarized antenna with a narrow 3dB beam-width of 25degrees and on bore-sight gain of 15dBiC. It also houses the necessary drive electronics and structural elements to provide a nimble, 2-axis antenna solution that meets the stringent mission requirements. The pointing accuracy is better than 1 degree and the maximum slew rate is better than 20deg/s, at acceleration rates up to 4deg/s/s. Such compact yet agile systems have not been used on small satellites before, but it permits the system designer to either maximise the downlink data throughput, or minimise the power resources on the spacecraft. As such, the APM is an enabling technology in the commercialization of small EO spacecraft missions. Numerous trades were made along the development path of the APM. The development was driven primarily by the high data rate required and the agility of the satellite, which required imaging and downlinking to be performed at the same time while minimizing the overall system cost. The high data rate forced the Equivalent Isotropic Radiated Power (EIRP) to increase to satisfy the RF Link Budget as increasing the Ground Station dish above 7.3m was considered far too expensive. As one of the other mission constraints was to minimize the DC power requirements it was not sensible to increase the XTX output power. Therefore the only other way to increase the EIRP was to increase the antenna gain. A simple Omni-directional antenna solution would provide a broad coverage which allows for off-pointing operation but the gain would be low and therefore could not support the high data rate required. A relatively simple isoflux antenna solution is geared towards providing a constant Power Flux Density (PFD) on the ground, but again the low gain capability limits the data rate. Additionally, due to the beam-shaped nature of the isoflux antenna off-pointing imaging while down-linking is not possible. More focused antennas such as a simple patch or horn antenna can provide much higher bore-sight gain capability. These are fine for closing the link budgets at the required data rate, but due to their inherent narrower beam-width they do not satisfy the off-nadir imaging and down-linking requirement for the mission. The satellite could be manoeuvred so the antenna bore-sight tracks the Ground Station during a pass, but this would limit the targets that could be imaged in the area of the Ground Station and it would place unnecessary demands on the attitude control systems of the satellite. The use of multiple high gain antennas and a switching matrix could satisfy the ability to image and down-link high rate data at the same time. However, due to the narrow beam-width of the individual high gain antenna a large number would be required. This would make the overall antenna solution potentially quite large and the control of the switch matrix could be complicated. The conclusion drawn after studying the capabilities of fixed antenna systems was that a steerable antenna system was needed to satisfy all aspects of the mission requirements, the salient points of the trade are captured in Table 1. Fixed Antenna Type Omni Isoflux Patch/Horn Switched Multiple Antenna Table 1: Fixed Antenna Trade-offs Pro s Simple Broad coverage Simple Constant PFD Simple Comments Con s Low gain Low gain Beam shaped Narrow beam Large Complex switching Once it was clear that a fixed antenna solution would not meet the mission requirements research focused on steerable antenna systems. The initial trade performed by SSTL was to determine if a mechanically steered or electrically steered antenna would best suit the requirements. Electrically steered antennas are typically very complex with multiple antennas and complex feed networks, the active variety can also be quite power hungry and expensive to develop. These systems can provide high gain and narrow beam-widths with fast response time for scanning the beam and the lack of moving parts means no issues with mechanical wear or micro-vibration. However, the range of beam movement is limited on flat arrays and the axial ratio at high off-nadir angles can be very poor. Mechanically steered antennas for space applications are typically designed to support medium to large dishes, but a small horn antenna was shown to satisfy the high gain requirements of this mission. Mechanical systems can have issues with mechanical wear. However, by moving to a fixed antenna the gain, beam-width and axial ratio can all be guaranteed over the full range of movement, which is considerably larger than in the case of electronic arrays. Correct motor sizing and bearing design can ensure the antenna speed is suitable for use Sensible motor driving techniques and employing a mechanically balanced design approach for the housing can ensure any micro-vibration generated is kept to a minimum. The DC power required was also expected to be low for the small unit needed to satisfy the requirements. It was also considered that a mechanically steered system would be much easier to scale in size to accommodate a larger, higher gain horn or even a dish antenna by scaling the required motors, bearings and housing. Equally it was considered that a single axis of a mechanically steered system could, without significant effort, be re-configured for use as a Garner 2 23 rd Annual AIAA/USU

3 Solar Array Drive Mechanism (SADM). The conclusion drawn after studying the capabilities of electrically versus mechanically steered antenna systems was that a mechanically steered system was best suited to satisfy all aspects of the mission requirements and future aspirations at SSTL, the salient points of the trade are captured in Table 2. Table 2: Electrically Steered Versus Mechanically Steered Antenna Systems Trade-offs Antenna System Electrically Steered Mechanically Steered Pro s Agile beam scanning No moving parts No microvibration issue Capable of required speed Low DC power Fixed axial ratio at high off-nadir angles Excellent pointing range Scalable for future needs Comments Con s Complex Expensive Potentially high DC power Limited pointing range Poor axial ratio at high off-nadir angles Mechanical wear issue Microvibration issue With a mechanically steered solution in mind, SSTL investigated the current market to confirm if there was anything suitable that was readily available in the price range of the mission. The conclusion was there was no such unit available. It was also considered that the alternative approach of buying in individual actuators and drive elements and assembling them in-house along with implementing an RF and DC path solution in addition to the mechanical solution would be too expensive. The alternative solution of handing this task over to a third party and only managing the interface aspects would also be very expensive. Additionally it did not follow the SSTL ethos of controlling risk on projects by managing the risks in-house if possible. SSTL proceeded to generate a number of options for a mechanically steerable antenna using either a patch or horn antenna. These options were derived as a result of examining the specific requirements as well as the design drivers. The key design drivers were: Cost Mass Pointing range Micro-vibration Modularity No launch lock As the NigeriaSAT-2 spacecraft is highly agile the mechanism needed at least 2 degrees of freedom and the total pointing range needed to satisfy the offpointing capabilities of the satellite, meaning the coverage needed to be greater than a hemi-sphere. The mass needed to be minimized to be consistent with the overall spacecraft low-mass approach. A modular design would allow the mechanism technology to be adapted to other applications, thus making good use of the investment into the development. The design needed to take micro-vibration into consideration from the start as any micro-vibration generated by the mechanism can directly affect the quality of the images. This is especially relevant in high resolution imaging missions. In order to minimize cost, the design made use of Commercial off the Shelf (COTS) parts and economic processes wherever possible. The desire to have no launch lock was mainly driven by the anticipated cost, complexity and risk of including one in the design. Having an ITAR free design removes any restrictions that could be placed upon the choice of end customer. All the designs also needed to consider two other areas of detailed design: RF path through the mechanism Transfer of electrical power and signals to and from the second axis of the mechanism, over the first axis The APM requires an RF co-axial connection between the output of the XTX and the antenna. A solution using a single length of coaxial cable gives the best performance in terms of RF losses but the trade-off between having long runs of cable for flexibility and concerns over life when flexed at the operational temperature extremes was considered. It is anticipated that over life the APM could experience in excess of one hundred thousand operation cycles for some high access missions. On balance, RF rotary joints were chosen as the lower risk option due to their better lifetime capability. However, higher path loss inside the APM of 1.2dB versus an anticipated 0.7dB is the result. A solution was also required to enable power and signal transfer across the first axis of the mechanism to the second. The preferred solution was a simple cable wrap contained within the mechanism. This enabled azimuth rotation up to 540degrees but did not require the expensive use of slip ring technology. At the end of the first stage of the development there were three concepts under consideration, as shown in Figure 2. These were: X-Y (XY) scheme Azimuth-Elevation (AE) scheme Swashplate scheme Garner 3 23 rd Annual AIAA/USU

4 X-Y AE SWASHPLATE Figure 2: The Three Mechanism Options Considered After Initial Design Phase After further investigation it was shown that the XY scheme did not satisfy the required pointing range so this option was rejected. The AE scheme, shown in Figure 3, and the Swashplate scheme, shown in Figure 4, were both manufactured to Engineering Model (EM) level to reduce risks to the mission and they were both subjected to life testing. The main reason for this decision was that both concepts included items with unverified lifetimes. The key areas of concern were: The RF rotary joints The cable wrap which transmitted power and signals through the first axis of the mechanism Lubrication of the motors Only one of the two mechanisms completed the life-test successfully. The AE mechanism suffered from an unexpected mechanical failure in the drive chain, eliminating the use of this type of drive-train. The Swashplate mechanism was also subjected to qualification level vibration, which it passed successfully. The final stage of the development was to optimise the design based upon everything that had been learnt during the program and generate the Qualification Model design. During the re-design process it was concluded that the best solution was in fact an Azimuth-Elevation type mechanism but using the modular approach and key components of the Swashplate design including smaller motors, bearings and gear modules as they were more suited to the job. Also the RF rotary joints and DC cable wrap had been tested so there was confidence in using them in the final design. The balancing of both axes, which was vitally important to avoid the need for launch lock and minimizing micro-vibration was also performed. This gave the best compromise between mechanism mass and control strategy while obtaining the required performance. Figure 3: Azimuth-Elevation Scheme Engineering Model Figure 4: Swashplate Scheme Engineering Model Garner 4 23 rd Annual AIAA/USU

5 The Qualification Model, shown in Figure 5 has been environmentally tested, including vibration and thermal vacuum life-test which meets all the NigeriaSAT-2 mission requirements. This meant performing almost 30,000 cycles to simulate 4 downlink contacts per day over the mission life of 7.5 years allowing for qualification margins. Additional life-testing is currently ongoing. This is to show the unit is capable of supporting missions that require multiple Ground Stations. As a result, the APM will see more mechanical cycles during the operational lifetime. At the time of writing, over 100,000 cycles have been performed successfully without degradation of performance and more are planned. be either left or right hand circular polarization during alignment of the unit prior to launch. The pointing accuracy is better than 1 degree and the maximum slew rate is better than 20deg/s, at acceleration rates up to 4deg/s/s. The range of movement is ±270degrees in azimuth and ±114.7degrees in elevation. This capability is housed inside a compact body with static dimensions of 272mm height and 196.2mm maximum width, with an interface plate diameter of 185mm. These dimensions also include the drive electronics which would typically sit below the mounting plane and therefore inside the satellite. The total mass, including drive electronics and antenna is 2.7Kg and the unit draws approximately 3.4W with both axes moving. Red ---- Co-Polar, Blue ---- Cross-Polar Figure 6: AE-90 Typical Co- and Cross-Polar Gain Measurements at 8.2GHz with cuts every 90degrees. Figure 5: AE-90 APM Qualification Model The finished APM incorporates a very simple horn antenna that is circularly polarized and provides a gain of better than 15dBiC on boresight with a 3dB beamwidth of approximately 25deg. Some typical coand cross-polar gain measurements taken on site at the SSTL facility at 8.2GHz are shown in Figure 6. The axial ratio is typically better than 3dB over the whole beamwidth and the insertion loss of the RF path inside the APM is around 1.2dB. The polarization can be set to POLARISATION DIVERSITY NigeriaSAT-2 actually has dual polarization capability with one APM antenna configured for left-hand polarization and the other APM antenna configured for right-hand polarization. SSTL typically employs cold redundancy for the payload downlink and a single PDC transmitting at 105Mbps is enough to satisfy the baseline mission requirements for data throughput. Historically at SSTL the cold redundant antennas would be designed for the same polarization to minimize the cost of the Ground Station dish feeds and satellite design costs. However, when a Ground Station is configured with both polarizations on the feed the possibility of making use of the polarization diversity to double the possible data throughput becomes worth investigating. The NigeriaSAT-2 mission Ground Station will have a dual polarized feed. The modular design of the APM horn antenna provided the capability Garner 5 23rd Annual AIAA/ USU Conference on Small Satelllites

6 of the APM to transmit in either left-handed or righthanded circular polarization with no impact on satellite cost. Achieving a good axial ratio on the circularly polarized satellite and Ground Station antennas is the key to minimizing the interference between the two polarizations when operating both transmission chains at the same time. Performance tests conducted on the NigeriaSAT-2 flight unit X-Band Transmitters and APMs have shown that the interference effects when operating both downlink chains are small and acceptable for the mission. The test configuration is shown in Figure 7, and all tests were performed inside a small anechoic chamber to remove the interference from reflections by lab equipment and metal objects. BER double the data throughput from an EO mission where obtaining time critical data in response to an international disaster could make all the difference to directing international aid agencies on the ground. 1.00E E E E E E E-08 Dual Polarisation 105Mbps QPSK with coding BER 1.00E E-01 To Receiver System ~12degrees off-nadir Rx Antenna (LHC) 1.00E-09 Ideal Uncoded QPSK XTX0-APM0 with XTX1 OFF - Baseline 1.00E-10 XTX0-APM0 with XTX1 ON Rx 0deg Rotated XTX0-APM0 with XTX1 ON Rx +90deg Rotated 1.00E-11 XTX0-APM0 with XTX1 ON Rx -90deg Rotated 1.00E-12 Eb/No (db) Figure 8: Dual Polarization Test Results X-Box FM APM1 (RHC) FM XTX1 2 4 R τ K t = 1 k 1 c a + tan ν δ Figure 7: Dual Polarization Test Configuration Tests were performed by obtaining baseline Bit Error Rate (BER) performance for the primary downlink chain without the secondary chain operating. The secondary chain, transmitting on the opposite polarization but with the same output power to simulate how it will operate orbit, was then powered up and the BER performance was again measured. In this case the receiving antenna was a test horn antenna which has an axial ratio that is worse than the Ground Station feed axial ratio is expected to be, so the performance of the operational system is expected to be better than shown in Figure 8. Additional BER tests were performed by rotating the receiving antenna about it s boresight to emulate how the satellite antenna and Ground Station antennas relative angles could vary during operation. It shows that the BER degradation can be a little higher depending upon the relative angles of the two antennas and again this will be related to the quality of the axial ratio of both antennas. A degradation of less than 1dB was observed during this testing and this is a very small overhead compared to the benefit of being able to FM APM0 (LHC) FM XTX0 850mm CONCLUSIONS SSTL has developed a compact yet agile 2-axis Antenna Pointing Mechanism that is free of ITAR parts which can support high capacity and high resolution EO missions aboard the SSTL-300 class of satellite. However, the APM would equally be suited to any Low Earth Orbit mission that requires a high-gain yet agile antenna solution. After launch in late 2009, the NigeriaSAT-2 mission will be capable of operating two downlink chains in parallel with the antennas transmitting in opposite circular polarization to enable the data throughput to be doubled compared to a traditional single string downlink chain. Compared to a lower gain fixed omni-directional antenna solution the improvement in gain of approximately 12dB by use of the APM allows the mission system designer to trade data rate, DC power requirement and Ground Station dish size. Compared to an omni-antenna the data rate can be increased by a factor of around 20. The dish size of the Ground Station could be reduced by a factor of 4 and still receive the same data rate. While the DC power requirement could be significantly reduced on the Transmitter by removing some power amplification stages and still retaining the same data rate. These benefits can obviously be improved when polarization diversity is also used. References 1. A. Cawthorne, M. Beard, A. Carrell, G Richardson, Launching 2009: The NigeriaSAT-2 mission high performance Earth observation with a small satellite, 22 nd Annual AIAA-USU Small Satellite Conference, Logan, Utah, August Garner 6 23rd Annual AIAA/USU